In a typical coal-fired power plant, coal is burned in a boiler to make high temperature and pressure steam to drive a steam turbine and electricity generator. For the steam-side process, the initial superheated steam drives the high-pressure turbine first for power generation, before returning to the boiler for reheating to bring steam temperature back to over 540° C. (according to the steam cycle specifications and boiler design). The reheated steam then enters intermediate-pressure and low-pressure turbines to generate additional electricity. The steam pipe connecting the intermediate-pressure turbine exhaust and the low-pressure turbine inlet is called the cross-over section, where the steam for solvent regeneration in the CO2 capture process is extracted. The exiting saturated steam from the low-pressure turbine is condensed in a water-to-steam heat exchanger (or condenser). The condensate is pressurized via feedwater pumps, heated via feedwater heaters and economizers, and fed back to the boiler to complete the steam cycle.
For the gas-side process, typically after combustion of the coal in low NOx burners the combustion gases leave the boiler and are treated in a NOx removal device called a Selective Catalytic Reduction (SCR). After the SCR treatment, the gas is further treated in a fly ash removal device, such as an electrostatic precipitator, to remove particulates. After this treatment, the gas is routed through an SO2 removal device (SO2 scrubber). At this point, the carbon capture process begins.
It is known in the art to use scrubbing solvents for capturing CO2 from post-combustion gases, such as from utility flue gases. As an example, a conventional solvent often used is 30 weight percent monoethanolamine (MEA). Likewise, it is known to use solvents comprising K2CO3/KHCO3 or NH3. Such conventional solvents, while generally effective for their intended purpose, can be markedly improved in terms of CO2 adsorption and recovery, required operating temperatures, energy requirements, and the like.
To solve the aforementioned and other problems, the present disclosure provides a mixed solvent including an amine and a low fraction of ammonia for CO2 removal, and further provides a stripping carrier having a low latent energy for solvent regeneration. Likewise, herein is disclosed a process for using the mixed solvent/stripping carrier in post-combustion CO2 capture. This mixed solvent provides multiple advantages, including: (1) higher mass-transfer flux which results in a smaller absorber to capture the same amount of CO2 from the flue gas stream; (2) higher carbon capacity, which reduces the liquid recycling rate between the absorber and the stripper, increases CO2 dissociated partial pressure and reduces the stripper size; and (3) less energy demand for CO2 stripping. The ammonia (i) reduces sulfur dioxide (SO2) levels in the pre-treatment tower to a suitable concentration to minimize MEA degradation; (ii) acts (in vapor form) as a carrier gas which reduces CO2 vapor pressure in the stripper; and (iii) enhances reaction kinetics and increases the solvent capacity.